The present invention relates to a method to reduce distortion of digital data by a Plain Old Telephone Service (POTS) ringing signal and a telephone ringing generator enabling this method.
Several methods and devices to reduce distortion of digital data, e.g. transferred in accordance with the ADSL (Asymmetric Digital Subscriber Line) standard specification, by the high voltage ringing of a POTS (Plain Old Telephone Service) signal, transferred simultaneously with the digital data over a single line, are known in the art. Traditional splitters consisting of active or passive high pass and low pass filters, such as the ones described in the article xe2x80x98ADSL and VADSL Splitter Design and telephony Performancexe2x80x99 from the authors John Cook and Phil Sheppard that was published in IEEE Journal on Selected Areas in Communications, Vol. 13, No. 9, December 1995, separate telephony signals (including ringing) from ADSL signals at the central office and at the customer""s premises thus minimising interference between the POTS and digital data transmission. These splitters however are bulky and expensive because high order (up to 7th order) operator dependent low and high pass filters are required therein.
In the full digital loop known for instance from the European Patent Application EP 0 740 451 entitled xe2x80x98Method, Interface Modules and telephone Network for Multiplexing and Demultiplexing an Analog MTS (Message Telephone Service) signal and an ADSL (Asymmetric Digital Subscriber Line) Datastreamxe2x80x99 from applicant Alcatel Bell, mutual interference between concurrently transferred telephony signals and digital data is avoided by digitising the telephony signals and by embedding the digitised telephony signals in the digital data frames that also convey the digital data. POTS signalling such as the ringing signal and the metering pulses are replaced with digital codes, also embedded in the digital data frames. These digital codes do not distort the digital data but encoding thereof at the transmitter""s side and decoding thereof at the receiver""s side requires some modifications of the current xDSL implementations. Moreover, the full digital loop requires by-passing equipment to maintain telephony service even when the digital data equipment fails, which can occur for instance in case of a power supply failure at the customer""s premises, and requires the presence of a ringing signal generator at the customer""s premises to re-generate the high voltage ringing signal from the received codes.
A third known method to reduce interference between simultaneously transferred digital data and telephony signals, in particular ringing signals, is known as the ADSL Lite solution with microfilters. In the ADSL Lite solution, studied for instance in the presentation summary xe2x80x98Study of Non-Linear Distortion in an ADSL-Lite Environmentxe2x80x99 written by the authors Marc Van Bladel and Gerd Vandersteen and presented on the 5th Bayona Workshop on Emerging Technologies in Telecommunications held in Bayona, Spain from 6 till 8 September 1999, microfilters in the line towards the phone separate the digital data from the telephony signals before these telephony signals are supplied to the phone. Such a microfilter for example is known from the press release xe2x80x98Orckit Announces New Dynamic Microfilter for Use with G.Lite ADSLxe2x80x99 from Jun. 2, 1999 that can be retrieved from the Internet via the following URL: http://www.xdsl.com/newsreleases/xDSL/3075.asp. The telephony signals however are not separated from the digital data before the digital data are supplied to the ADSL modem so that there is distortion of digital data by the ringing signal. As a consequence, the digital data service usually goes down in the ADSL Lite solution at the moment a telephone call is set up over the same twisted pair.
An object of the present invention is to provide a method and a device for reducing distortion of digital data by the POTS ringing of a telephony signal transmitted concurrently with the digital data over a single line, but which overcome the drawbacks of the above mentioned state of the art solutions, i.e. which do not require bulky splitters with 7th order filters at the central office and customer""s premises, which do not require additional encoding/decoding complexity and precautions to realise a POTS life-line in case of failure of the digital data service, and which avoid that the digital data service can go down as a result of a presence of the high voltage POTS ringing signals.
According to the invention, this object is achieved by the method to reduce distortion of digital data by a Plain Old telephone Service (POTS) ringing signal defined by claim 1, and the telephone ringing signal generator defined by claim 2.
Indeed, by windowing a sine or cosine shaped signal, a digital data friendly ringing signal is generated. The influence of the on/off switching of this digital data friendly ringing signal on digital data concurrently transmitted over the same line is far below the transient distortion due to on/off switching of a traditional sine or cosine shaped ringing signal. When smoothly windowing a sine or cosine shaped signal to constitute a digital data friendly ringing signal, a slight increase of the peak-to-average ratio (PAR) of the ringing signal is an acceptable price paid for the obtained reduction in transient distortions.
It is to be noticed that the term xe2x80x98comprisingxe2x80x99, used in the claims, should not be interpreted as being limitative to the means listed thereafter. Thus, the scope of the expression xe2x80x98a device comprising means A and Bxe2x80x99 should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.
An optional characteristic feature of the telephone ringing signal generator according to the present invention is defined by claim 3.
In this way, for new operators that do not have to care about an installed base (the so called green field installation), a per-line scenario can be implemented with a universal linecard that terminates both the telephony service and the digital data service and with an intelligent switch, smoothly controlled by for example a microprocessor so that the on/off switching of the ringing signal is aligned with respectively the beginning/ending of the applied window. For this implementation of the present invention, a traditional ringing generator sourcing an analog sine or cosine shaped ringing signal can serve as input for the smoothly controlled switches on the different linecards.
An additional characteristic feature of the telephone ringing signal generator of claim 3, is defined by claim 4.
Thus, in order to avoid distorting the ringing signal heavily by the windowing applied to reduce the ringing signal transient distortion on digital data, the window length may be selected to be equal to the on-period of the ringing signal.
Alternatively to the telephone ringing signal generator described by claim 3, an optional characteristic feature of the telephone ringing signal generator according to the present invention is defined by claim 5.
In this way, for incumbent operators that offer digital data services on top of telephony services and that want to reuse to a maximum their installed infrastructure, a scenario can be implemented whereby only the ringing card is updated to a ringing card that generates the windowed digital data friendly ringing signal, and whereby all POTS linecards and switches for on/off switching the ringing signal on the different lines are reused.
An additional characteristic feature of the telephone ringing signal generator of claim 5, is defined by claim 6.
Thus, the reused switches for on/off switching the ringing signal are controlled, for instance by a microprocessor that communicates with the ringing card generating the windowed digital data friendly ringing signal, to switch on/off respectively aligned with the beginning/ending border of the applied smooth window. The transient distortion of digital data due to on/off switching of the ringing signal is minimized this way.
Another additional characteristic feature of the telephone ringing signal generator of claim 5, is defined by claim 7.
Indeed, the window size has an impact on the reduction of transient distortion of digital data by the ringing signal. In general, a longer window size results in better transient distortion reduction. In practice, the maximum window size is limited due to the ringing cadence to the length of the on-period of the ringing signal. One on-period of the ringing signal may however comprise more than one window. An implementation wherein the window is applied more than once per on-period of the ringing signal (the window may for example have the length of 2 sine or cosine periods) may comply with existing timing specifications for switching off the ringer after ring trip detection even if the system is designed to delay on/off hook transitions until the next border of the window in order to reduce disturbance of the digital data transmission due to switching off the ringer after ring trip detection. This is not realizable with a window having the length of the full on-period of the ringing signal, because the existing delay specifications for POTS do not allow to postpone off hook until the end of the on-period of the ringing signal. The ringing signal itself however is more distorted when applying a shorter window repetitively during the on-period of the ringing signal.
A particular implementation of the telephone ringing signal generator of claim 7, is defined by claim 8.
In this way, by repetitively applying a smooth window having a length of only one sine or cosine period, the transient distortion of simultaneously transferred data due to on/off switching of the ringing signal is optimally suppressed. As already indicated in the one but previous paragraph, this optimal transient effect reduction is obtained at the price of a higher distortion (deviation from the traditional sine or cosine shaped ringing signal) of the ringing signal itself.
Furthermore, an advantageous characteristic feature of the telephone ringing signal generator according to the present invention, is defined by claim 9.
Indeed, starting windowing in a zero transition of the sine or cosine shaped signal reduces transients by up to 50 dB at the low frequency edge of the digital data band (on the lowest tones of a Discrete Multi Tone signal if DMT is used as modulation technique for the digital data). In order to minimize distortions the slope of the ringing signal must be smooth. Therefore, the window has to start and end smoothly. Because the peak-to-average ratio (PAR) of the ringing signal should not increase too much, there is also an upper bound for the smoothness or length of the rising and falling edge of the window. Taking into account that the ringing signal starts at a zero transition, the just described trade-off is met if the rising edge of the window starts at zero phase and ends between xcfx80/2 and xcfx80 phase of the sine or cosine shaped signal. In general it is concluded that optimum transient effect suppression is obtained with a flat starting and ending window, whereby the window size is large enough in order to keep the slope of the ringing signal within reasonable boundaries.
Particular examples of the telephone ringing signal generator of claim 9, are defined by claims 10 and 11 respectively.
Indeed, a Hanning window as well as a polynomial window are smooth windows that start at zero amplitude and whose rising edge can be shaped to start at zero phase of the sine or cosine shaped signal and ends in between xcfx80/2 and xcfx80 phase of this sine or cosine shaped signal.